Selective catalytic reduction (SCR) provides a method for removing nitrogen oxides (NOx) emissions from fossil fuel powered systems for engines, factories, and power plants. During SCR, a catalyst facilitates a reaction between a reductant (e.g. ammonia) and NOx to produce water and nitrogen gas, thereby removing NOx from the exhaust gas. Generally, the exhaust gas ammonia is mixed with an exhaust gas stream upstream of an SCR catalyst. The ammonia may be injected directly into the exhaust gas stream or supplied as urea, which can be converted to ammonia under appropriate conditions.
To optimize NOx removal from the exhaust gas stream, while preventing release of potentially noxious ammonia, the NOx and ammonia are generally supplied at approximately one-to-one ratios. However, it may be difficult to match the quantities of ammonia and NOx produced by on-board ammonia production systems and a NOx source, such as an engine. Further, the flow rates between the ammonia production system and SCR catalyst and the NOx source and SCR catalyst may vary. Therefore, even if ammonia production and NOx production are appropriately matched, the ammonia and NOx concentrations at a downstream SCR system may be mismatched. Improved SCR systems that provide better matching of ammonia and NOx concentrations at the SCR catalyst are needed.
One method for controlling exhaust gas reductant and NOx concentrations for SCR is disclosed in U.S. Pat. No. 6,092,367, which issued to Dölling on Jul. 25, 2000 (hereinafter the '367 patent). The method of the '367 patent includes introducing a quantity of a reducing agent and setting the quantity of the reducing agent. The quantity is set using a functional relationship based on operationally-relevant parameters of a combustion installation, an exhaust gas, and a catalytic converter. The method may further include adapting the functional relationship to a current state of the catalytic converter.
Although the method of the '367 patent may provide suitable control of reductant and NOx concentrations, the method of the '367 patent may have several drawbacks. For example, the method of the '367 patent may be suitable for systems with on-board storage of ammonia or other reductants in which the reductant may be supplied proximate the SCR catalyst. However, the method of the '367 patent does not take into account variations in flow rates of ammonia and NOx, as may occur with on-board ammonia production systems.
The present disclosure is directed at overcoming one or more of the shortcomings of the prior art SCR systems.